staging: comedi: jr3_pci: tidy up jr3_pci_poll_dev()

For aesthetics, rename the subdevice private data pointer from 'subdevpriv'
to 'spriv' and add a local variable for the comedi_subdevice pointer.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>